docs: update TEST_SPEC.md and README.md to reflect new directory structure

- TEST_SPEC.md: Updated test directory structure to reflect Core/Asset,
  Core/IO, and Resources/<Type> subdirectories
- TEST_SPEC.md: Updated module names and test counts (852 total)
- TEST_SPEC.md: Updated build commands for new Resources subdirectories
- README.md: Updated engine structure with Core/Asset/ and Core/IO/
- README.md: Updated Resources section with layered architecture
- README.md: Updated test coverage table with accurate counts

tests/Components/test_game_object.cpp

@@ -2,7 +2,7 @@
 #include <XCEngine/Components/GameObject.h>
 #include <XCEngine/Components/TransformComponent.h>
 #include <XCEngine/Scene/Scene.h>
-#include <XCEngine/Math/Vector3.h>
+#include <XCEngine/Core/Math/Vector3.h>
 
 using namespace XCEngine::Components;
 using namespace XCEngine::Math;

tests/Components/test_transform_component.cpp

@@ -1,6 +1,6 @@
 #include <gtest/gtest.h>
 #include <XCEngine/Components/TransformComponent.h>
-#include <XCEngine/Math/Math.h>
+#include <XCEngine/Core/Math/Math.h>
 
 using namespace XCEngine::Components;
 using namespace XCEngine::Math;